I look back and shudder to think how many years before diagnosis William was showing signs of dementia! There were odd incidents going back many years, and changes in behaviour and moods. I knew he had dementia for around 3 years before his GP tentatively diagnosed him in 2008, saying he was "pretty sure" - but William was still scoring perfectly on tests in 2011 when we managed to get him an emergency appointment with a Consultant because of a psychotic breakdown (caused by the dementia). He continued to score very highly for about another 1 1/2 years, as his dementia very obviously progressed. Then there was a sudden and dramatic drop.
His consultant told me once that although every patient is very different because so many other aspects of their health & lifestyle will influence things, in very general terms, for someone William's age (77 when diagnosed by GP in 2008), expected life span would probably be in or around ten years from diagnosis. But he did qualify that to say that we had to bear in mind that we couldn't be sure how long William had had the illness, as his very high intellectual & educational level had masked it for so long - and also that it really depended on how well the person remained. Some became more prone to infections than others, which took a severe toll on their health. William had always been prone to respiratory infections and was asthmatic - and eventually succumbed to aspiration pneumonia at the age of 84, seven years after initial diagnosis.